1. Field of the Invention
The present invention relates to a layered structure in which a nitride compound semiconductor film of an excellent quality can be obtained, and a method for forming the same.
2. Description of the Related Art
Conventionally, a nitride compound semiconductor has been employed and studied as a light-emitting diode and a device with a high-temperature resistance. By adjusting its composition, a nitride compound semiconductor can be employed as a light-emitting diode for a wide range of short wavelengths from blue to orange.
It has been known in the art that there is a need to reduce threading dislocation and cracks in a crystal for realizing a nitride compound semiconductor with excellent characteristics and a high reliability. As a conventional method for reducing the threading dislocation and cracks, the following method has been suggested (Jpn. J. Appl. Phys., Vol.36 (1997), pp. L899-L902). First, a thin film of GaN is epitaxially grown on a substrate by a metal organic chemical vapor deposition (MOCVD). After a striped selective growth mask of SiO2 is formed on the substrate as a growth suppressing member, GaN is epitaxially grown on the wafer. According to this method, a flat epitaxial film is formed due to lateral crystal growth which occurs over the selective growth mask.
FIGS. 7A to 7D illustrate the steps of the above-mentioned method. A thin GaN film 702 with a thickness of about 0.5 xcexcm to about 2 xcexcm is deposited on a sapphire substrate 701 via a low-temperature buffer layer (not shown) of a GaAlN type material (FIG. 7A). Next, a SiO2 film is formed on the thin GaN film 702, and patterned by a common photolithography technique so as to provide a striped SiO2 selective growth mask 703 (width: about 5 xcexcm, pitch: about 7 xcexcm) (FIG. 7B).
Thereafter, a thick GaN film 704 with a thickness in the range of about 10 xcexcm to about 300 xcexcm is deposited by a hydride vapor phase epitaxy (HVPE) or MOCVD method. At the initial growth stage of the thick GaN film 704, the GaN crystal grows only in a window area 704 of the thin GaN film 702 on which no selective growth mask 703 is formed, as shown in FIG. 7C. At this stage, crystal deposition is locally suppressed by the selective growth mask 703, thereby selectively growing the crystal as shown in FIG. 7C. However, as the GaN crystal continues to grow, the GaN crystal on the window region 704 starts to laterally extend over the selective growth mask 703 (in this specification, this will be referred to as xe2x80x9clateral growthxe2x80x9d). As a result, GaN crystals growing from adjacent window regions 704 attach to each other, thereby forming a thick GaN film 705 exhibiting a single layer structure (FIG. 7D).
Curve 201 in FIG. 2 illustrates density of threading dislocation on a wafer surface for various thicknesses (about 10 xcexcm, about 50 xcexcm, about 100 xcexcm, and about 300 xcexcm) of the thick GaN film 705 obtained by the conventional method. Threading dislocations are evenly distributed in all regions of the thick GaN film 705. The density is reduced as the thickness increases, and with the thick GaN film 705 having a thickness of about 100 xcexcm or more, the density of threading dislocations is reduced to about 5xc3x97107 cmxe2x88x922. No crack is observed in the thick GaN film 705 irrespective of the thickness thereof.
However, the conventional method and the thick GaN film 705 obtained by the conventional method have the following problems.
(1) It is impossible to reduce the density of threading dislocation on the surface of the thick GaN film 705 to a density on the order of 106 cmxe2x88x922 or less as needed for a device used with a large current density, such as a light-emitting diode and particularly a semiconductor laser device. Thus, if a GaN-type semiconductor laser device is formed on the wafer as shown in FIG. 7D, the operating life of the device will be as short as about 400 hours (under conditions of 60xc2x0 C. and 5 mW), failing to realize the commercially required operating life of about 5000 hours.
(2) The manufacturing method is tedious because it is necessary to perform two epitaxial growth steps for the thin GaN film 702 and for the thick GaN film 705.
According to one aspect of this invention, a method for forming a nitride compound semiconductor film includes the steps of: (1) providing a substrate having a portion which acts as a growth suppressing film on a outermost surface thereof; (2) forming a growth promoting film partially on the substrate; and (3) forming a nitride compound semiconductor on the growth promoting film.
In one embodiment of the invention, the growth suppressing film is an amorphous film.
In another embodiment of the invention, the substrate is a crystal substrate of a cubic crystal structure having a surface along a (110) orientation or a (110) orientation.
In still another embodiment of the invention, the growth promoting film is ZnO or InsGawAl1xe2x88x92sxe2x88x92wN (0xe2x89xa6sxe2x89xa61, 0xe2x89xa6wxe2x89xa61, and 0xe2x89xa6s+wxe2x89xa61).
In yet another embodiment of the invention, a thickness of the growth promoting film is equal or greater than about 0.2 xcexcm.
In another embodiment of the invention, the growth promoting film is in a form of a plurality of stripes separated from one another by an interval of about 20 xcexcm or less.
According to another aspect of the present invention, a nitride compound semiconductor light-emitting diode includes: a substrate having a portion which acts as a growth suppressing film on a top surface thereof; a growth promoting film formed partially on the substrate; and a nitride compound semiconductor layered structure including a light-emitting layer formed on the substrate, the light-emitting layer having a light-emitting region into which an electric current injected. The light-emitting region is formed above a region of the substrate where no growth promoting film is formed.
Thus, the invention described herein makes possible the advantages of (1) providing a convenient method for forming a GaN-type semiconductor layer with a low density of threading dislocations, and (2) improving the operating life of a semiconductor laser device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.